This invention relates to motor drive systems using current source power converters, and more particularly to a motor controller having provision for the dynamic braking of an ac machine fed from a controlled current inverter.
During regenerative braking of a conventional voltage and frequency controlled induction motor, the direction of current through the inverter supplying the motor reverses and power is returned to the dc supply. Controlled current drive systems, however, use a current-fed inverter in which the output current magnitude and frequency are controlled rather than the output voltage and frequency as in the usual inverter. The dc reactor in cascade with the current-fed inverter forces constant dc link current which is not easily interrupted. Although the rectifier output voltage and inverter input voltage change polarity during regenerative operation, the direction of current does not change polarity. Accordingly, different techniques are needed to control operation of the current source power converter while braking, particularly when ordinary regenerative braking is not suitable such as when the ac line will not accept the power fed back to it. In large traction drives, for example, dynamic braking is highly desirable. In this case power can be dissipated by opening the ac line and placing a braking resistor across the rectifier output terminals, but in the absence of further controls the braking action is uncertain and can result in motor instability. Thus, a dynamic braking control of any regulated drive should provide for control of the power dissipated in the braking resistor in order to assure constant torque control during braking.